This invention relates to a heat developable photosensitive material and, more particularly, to a heat developable photosensitive material for scanners of image setters suitable for photomechanical processes. More specifically, this invention relates to a heat developable photosensitive material for photomechanical processes obtainable of images less subject to fog and having a high Dmax (maximum density).
A large number of photosensitive materials having a photosensitive layer on a support for forming images upon imagewise exposure have been known. Among them, as a simplified system for rendering preservation of environments and image forming means, a technology for forming images by heat development is exemplified.
In recent years, reduction of the amount of waste processing solutions is strongly demanded in the field of photomechanical processes from the standpoint of environmental protection and space savings. To cope with this, techniques are needed in relation to photosensitive heat developable materials for use in photomechanical processes, which can be effectively exposed by a laser scanner or laser image setter and can form clear black images having high resolution and sharpness. Such heat developable photosensitive materials can provide to customers a heat development processing system, without use of solution-type processing chemicals. That is simpler and free from incurring environmental destruction.
Methods for forming an image by heat development are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075 and D. Morgan and B. Shely, Imaging Processes and Materials, xe2x80x9cThermally Processed Silver Systemsxe2x80x9d A, 8th ed., page 2, compiled by Sturge, V. Walworth and A. Shepp, Neblette (1969). The photosensitive material used contains a light-insensitive silver source (e.g., organic silver salt) capable of reduction, a photocatalyst (e.g., silver halide) in a catalytic activity amount, and a reducing agent for silver, which are usually dispersed in a reducing agent for silver, which are usually dispersed in an organic binder matrix. This photosensitive material is stable at room temperature. However, when it is heated at a high temperature (e.g. 80xc2x0 C. or higher) after the exposure, silver is produced through an oxidation-reduction reaction between the silver source (which functions as an oxidizing agent) capable of reduction and the reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image generated upon exposure. The silver produced by the reaction of the silver salt capable of reduction in the exposure region provides a black image and this presents a contrast to the non-exposure region. Thus, an image is formed.
The heat developable photosensitive materials have been known previously, but in most of those, the photosensitive layer is formed by coating a coating composition having a solvent of an organic solvent such as toluene, methyl ethyl ketone (MEK), methanol, and the like. Use of such organic solvents as a solvent not only adversely affects human bodies during manufacturing processes but also is disadvantageous in term of costs due to recycling the solvents.
To cope with this, a method has been considered in which a photosensitive layer (hereinafter referred also to as xe2x80x9caqueous photosensitive layerxe2x80x9d) is formed using a coating composition of a water solvent not having the above problem. For example, Japanese Unexamined Patent Publication (KOKAI) Showa (hereinafter referred to as xe2x80x9cJP-A-xe2x80x9d) Nos. 49-52,626 and 53-116,144, and the set forth an example that gelatin is used as a binder. Also, JP-A-50-151,138 sets forth an example that a polyvinyl alcohol is used as a binder.
In JP-A-60-28,737, an example that gelatin and polyvinyl alcohol are used together is described. In addition, as another example other than the above examples, JP-A-58-28,737 sets forth an example of photosensitive layer that a water-soluble polyvinyl acetal is used as a binder.
If such a binder is used, the photosensitive layer can be formed in use of a coating composition with a water solvent, and as for photosensitive silver halide, it is advantageous to add prescribed materials according to a conventional method.
However, if ordinary gelatin having a molecular weight of about 100,000 is used for dispersion state turns into a bad state in a coating composition containing the organic silver salt and may be agglutinated, thereby obtaining only products having considerably diminished values such that the blackened concentration at a light exposed section is low while the concentration at an unexposed portion is high.
Therefore, a technology is desired providing a heat developable photosensitive material capable of obtaining images with low fog, high Dmax (maximum density), and less fog increase and less sensitivity deviations during preservation, as well as having advantages in terms of environments and costs.
Accordingly, the first object to be accomplished by the invention is to provide a heat developable photosensitive material capable of obtaining images with low fog and high Dmax (maximum density), particularly suitable for photomechanical processes as well as for scanners or image setters, with a good coating surface shape.
The second object of the invention is to be solved is to provide a heat developable photosensitive material capable of coating with water with advantages in terms of environments and costs.
The objects are accomplished by the means below. That is, this invention is as follows:
[1 ] A heat developable photosensitive material having on a support a non-photosensitive sliver salt, a photosensitive sliver halide emulsion, and a binder, wherein the photosensitive sliver halide emulsion comprises a photosensitive sliver halide and a low molecular weight gelatin having a molecular weight of 500 to 60,000.
[2] The heat developable photosensitive material according to [1], wherein the low molecular weight gelatin has a molecular weight of 1,000 to 40,000.
[3] The heat developable photosensitive material according to [1] or [2], wherein the photosensitive sliver halide emulsion comprises a photosensitive sliver halide which is formed independently of the non-photosensitive sliver salt.
[4] The heat developable photosensitive material according to any one of [1]-[3], wherein the emulsion is prepared by the method comprising the step of adding the low molecular weight gelatin to a desalted composition containing photosensitive sliver halide particles.
[5] The heat developable photosensitive material according to [4], wherein the emulsion is prepared by the method comprising the steps of preparing sliver halide particles in the presence of a gelatin having a molecular weight of more than 60,000, and desalting the resulting composition, and then adding the low molecular weight gelatin to a desalted composition.
[6] The heat developable photosensitive material according to any one of [1]-[5], wherein the emulsion is prepared by the method comprising the step of preparing sliver halide particles in the presence of the low molecular weight gelatin.
[7] The heat developable photosensitive material according to any one of [1]-[6], wherein the low molecular weight gelatin is an alkali-processed gelatin, an acid-processed gelatin or a phthalic gelatin.
[8] The heat developable photosensitive material according to [7], wherein the low molecular weight gelatin is an alkali-processed gelatin.
[9] The heat developable photosensitive material according to any one of [1]-[8], wherein at least 50% by weight of the binder of an image forming layer containing the photosensitive sliver halide is a polymer latex having a glass transition temperature of xe2x88x9230xc2x0 C. to 40xc2x0 C.
[10] The heat developable photosensitive material according to [9], wherein at least 70% by weight of the binder of an image forming layer containing the photosensitive sliver halide is a polymer latex having a glass transition temperature of xe2x88x9230xc2x0 C. to 40xc2x0 C.
[11] The heat developable photosensitive material according to any one of [1]-[10], which further comprises a nucleation agent.